In Eberron, there is a feat called "Adamantine Body." This feat gives Warforged an armor bonus of +8, damage reduction 2/adamantine, -5 to armor check skills, only allows +1 to dex bonus to AC, and an arcane spell failure chance of 35%.

What the heck is damage reduction 2/adamantine? This is so foreign to me, it's not funny. Unfortunately, the campaign setting book is doing nothing for me. Anyone care to assist?

simple, if they are hit with anything that is not made of adamantine (which is a really rare and ultra-spiffy metal) then the first 2 points of damage are absorbed / reduced by the spiffy armor the warforged has.

I do not remember if adamantine is in the Eberron book or the DMG, but Damage Reduction is in the DMG and explains the whole thing:

Some magic creatures have the supernatural ability to instantly heal damage from weapons or to ignore blows altogether as though they were invulnerable.

The numerical part of a creature’s damage reduction is the amount of hit points the creature ignores from normal attacks. Usually, a certain type of weapon can overcome this reduction. This information is separated from the damage reduction number by a slash. Damage reduction may be overcome by special materials, by magic weapons (any weapon with a +1 or higher enhancement bonus, not counting the enhancement from masterwork quality), certain types of weapons (such as slashing or bludgeoning), and weapons imbued with an alignment. If a dash follows the slash then the damage reduction is effective against any attack that does not ignore damage reduction.

Ammunition fired from a projectile weapon with an enhancement bonus of +1 or higher is treated as a magic weapon for the purpose of overcoming damage reduction. Similarly, ammunition fired from a projectile weapon with an alignment gains the alignment of that projectile weapon (in addition to any alignment it may already have).

Whenever damage reduction completely negates the damage from an attack, it also negates most special effects that accompany the attack, such as injury type poison, a monk’s stunning, and injury type disease. Damage reduction does not negate touch attacks, energy damage dealt along with an attack, or energy drains. Nor does it affect poisons or diseases delivered by inhalation, ingestion, or contact.

Attacks that deal no damage because of the target’s damage reduction do not disrupt spells.

Adamantine is a magical material that is in both the 3.0 and the 3.5 DMGs. It has much the same properties in both editions, but in 3.5 it dovetails into the new Damage Reduction system.

In 3.0, DR is only a little bit different from the pre-3e rule of "+x or better to hit." In 3.0, a golem might have a Damage Resistance of +4/40 -- that is, unless you've got a +4 weapon, it ignores the first 40 points of your attack. Which, of course, for most characters, makes a +4 weapon necessary to hurt the golem at all.

In 3.5, a much better system was introduced, that better reflects the mythology and versimilitude of the setting. Raw magical energy is only relevent to a few creature's DR, where it is noted as simply "magic" or "epic". Most DR is keyed to a certain material -- cold iron, silver, an alingment, or adamantine. Also, DR has dropped in power, to no more than a DR of 15 for the strongest creatures -- enough to be difficult without the right weapon, but not impossible.

In that case, I suggest you browse the rules forum at www.enworld.org. There's lots of excellent discussion about the finer points of the rules there. You can also get good advice when things in your game go crazy :)